The present invention relates to a color image processing system and in particular to a color image processing system to which halftone image data having a plurality of colors is input for processing for drawing, which is preferable for applications in such as color printing machines.
In general, subtractive mixture of primary three colors is used for representing colors of prints. If C (cyan), M (magenta) and Y (yellow) of inks are mixed, black color would be theoretically obtained. However, in fact complete black color can not be obtained due to characteristics of inks. Accordingly, complete black color is obtained by provision of pure black ink. At this end, image data having four colors such as C, M, Y and K (black) would be required.
In order to draw multigradation image data, a processing method is conducted comprising, drawing the image data using primary three colors of subtracting mixture, thereafter reading the drawn image data of primary three colors for generating a black form, and reducing the gradation of C, M and Y forms depending upon the gradation of the generated K form.
However, since in accordance with the prior art method, determination on drawn and undrawn pixels could not be conducted if the pixels of all C, M and Y prints are read. The above-mentioned processing should be conducted for all pixels. There is a problem that an enormously extended period of time is taken for processing.
Japanese Patent Kokai Publication No. JP-A-90-294880 (Application No. Hei 9-116119) proposes a color images processing system which are capable of providing with high quality images which are equivalent to those provided by a system of processing of four colors. In the proposed system, image data of four CMYK colors having multigradation is generated by performing a BG/UCR (black generation/under color removal) conversion on the input multigradation image data of three RBG colors, and the generated four color image data is rasterized as binary data of three CMY colors under assumption based upon the binary average error minimization technique. The color tone of processing is not changed for any applications even in a four-color printer of the system, in which raster operation (ROP) is performed after the stage of average error minimization technique, such as printer for Windows. And thus, BG/UCR processing with multigradation data is possible. In this color image processing system, speeding up is achieved while keeping a high image quality by adopting a method of binarization of C, M and Y plates.
However, problems have been encountered during intense investigations toward the present invention. Namely, there is a problem that the upper limit of possible image quality is lowered at the time of binarization.
Various UCR/BG methods have been known. Various methods can be selected depending upon its processing speed and resultant image quality.
As mentioned above, prior art color image processing system has a problem that high speed drawing at the stage of drawing processing will deteriorate the color tone of the image while reproducing of color tone with high fidelity will rapidly lower the processing speed.
The reason resides in that for reproduction of color tone at high fidelity in the prior art, a K plate (data) should be generated after an entire ROP has been completed when ROP taking into account of the under color is performed. Processing of an algorithm for generation of the K plate is applied over the entire area to be drawn.
Therefore, the present invention was made ion view of above-mentioned problem. It is an object of the present invention to provide a color image processing system which achieves speeding up with an enhanced image quality.
Other objects and aspects will become apparent in the entire disclosure.
According to an aspect of the present invention, there is provided a color image processing system comprising:
input means for inputting multigradation image data having a plurality of colors; drawing means for conducting a raster operation (ROP) of image data input from said input means into a buffer memory; and output means for separating colors of the drawn image data into a plurality of colors and for outputting them. The system is characterized in that said system comprises means for updating an output image by performing a raster operation (ROP) based on read under image and new data to be drawn when drawing processing for a same pixel position should be performed a plurality of times.
Typically, the output multigradation image data on one color of said plurality of colors is the multigradation image data representative of black color.
In the present invention, the image data of black color is generated by performing a UCR/BG conversion for the output multigradation image data of the plurality of the other colors.
In the present invention, the multigradation image data of black color is reduced to the multigradation image data of a plurality of the other colors by performing a reverse UCR/BG (under color removal/black generation) conversion when the under image is read in said plural drawing processings and a raster operation (ROP) with new image data is performed.
In the present invention, in said plural drawing processings, an UCR/GB conversion is performed for an input of the newly generated multigradation image data having a plurality of colors, and the multigradation image data of black color is extracted again from the multigradation image data of a plurality of the other colors for generating and outputting the image data in which black color element is removed from the image data of the plurality of the other colors.